I've read it's typically 180 days (6 months), but is it usually counted from the date of purchase, or date of delivery (when I could actually check that the good is not fit for purpose?)
This depends on your local laws and your agreement with the bank issuer. It also depends on which network your issuer belongs to - Visa/Mastercard/Diners Club/UnionPay/JCB/American Express/etc etc - there is a bunch. Each has its own rules.
In your case, you found a Russian reference, while you're yourself in Armenia. You need to look for local rules. Call your card issuer and ask them.